Large Scale Dynamo in a Primordial Accretion Flow -- An Interpretation from Hydrodynamic Simulation

Abstract

Without an existing large scale coherent magnetic field in the early Universe, Population III (PopIII) stars would likely rotate at or near break-up speed. In this work, focusing on the accretion phase of PopIII stars, we investigate the possibility of generating a coherent magnetic field through large scale dynamo processes, as well as the corresponding field saturation level. Using results from hydrodynamic simulations, we demonstrate that primordial accretion disks are turbulent with a Shakura-Sunyaev disk parameter $\alpha_{ss} \gtrsim 10^{-3}$, and evidence helical turbulence with a dynamo number $\vert D_{\alpha \Omega} \vert \gg 10$. The presence of helical turbulence at these levels allows large scale dynamo modes to grow, and the saturation level is determined by the amount of net helicity remaining in the dynamo-active regions (a.k.a. the quenching problem). %We demonstrate that the magnetic field can reach approximate equipartition, with $B/B_{\rm eq} \sim 3$, indicating that the dynamo quenching problem could be alleviated through an accretion flow. We demonstrate that, if the accretion could successfully alleviate the quenching problem, the magnetic field can reach approximate equipartition with $B/B_{\rm eq} \sim 3$.